This invention relates generally to power amplifiers and more particularly to hybrid combining RF power amplifiers.
To meet the high RF output power requirements of radars, communication systems, etc. using transistor amplifiers, one approach to achieve that high power is to combine the output signals of a plurality of lower power transistor amplifiers using a network of quadrature hybrid couplers. An example is shown in FIG. 1 of U.S. Pat. No. 3,371,284 issued to Engelbrecht. Here, a first quadrature hybrid coupler splits an input signal into two signals having equal power in quadrature. These two signals couple to corresponding essentially identical amplifiers. Amplified signals from the output of these amplifiers are recombined by a second quadrature hybrid coupler. The vector components of the amplified signals that are in quadrature are summed to a first output port of the second quadrature hybrid coupler for coupling to an output load. However, the vector components of the amplified signals not in quadrature due to deviation by the amplified signals from quadrature are summed by the second quadrature hybrid coupler to a second output port (isolate) for coupling to a matched terminating load. The deviation from quadrature causes power from the amplifiers to be dissipated in the matched load instead of the output load. A mismatch by the output load presented to the output of the amplifiers by the second quadrature hybrid coupler will force this deviation from quadrature. This deviation from quadrature due to the mismatch to the outputs of the amplifiers is called phase-pulling. With class C type of high efficiency amplifiers, this phase-pulling effect for a given amount of mismatch is greater than with lower efficiency amplifiers, such as class A. To minimize the phase-pulling, the impedance match between the input to the second quadrature hybrid coupler and the output of the amplifiers must be tightly controlled. This impedance match is made very difficult with any output load mismatch. To aggravate matters, the phase-pulling occurs in opposite directions for each amplifier due to the quadrature phase splitting by the second quadrature hybrid coupler on signals reflected from the mismatched load. Use of an isolator at the output port of the second hybrid coupler will protect the amplifier from load mismatches, but an isolator is not perfectly matched itself, having a typical VSWR of 1.25 to 1. Additionally, the second quadrature hybrid coupler itself has a typical VSWR of 1.15 to 1, so that at a particular operating frequency, the combined VSWR of the isolator and second quadrature hybrid coupler would be 1.5 to 1, causing significant loss of the output power from the combined amplifiers to the output load, that loss of power being dissipated in the matched terminated load on the isolate port of the second quadrature hybrid coupler.